Freightliner Century S60 low boost! Help

Hi everyone, have some weird problem. I’ve got Detroit S60 14 L , and in one day I found it makes really low boost and poor MPG. So I decided to do a pressure charge air system leak test , and found major leaks over intake manifold gaskets . So I replaced them ,clean manifold and head with metal brush.Did test again and problem is still there . Basically system is not holding pressure ( pressure drops right away, from 25psi to 0 psi in 10 seconds) .Re did the test and found that air still leaking through intake manifold gaskets but much less . These gaskets just metal with no rubber or something like that . So should I try to put some sealer over the gaskets or what to do ? But honestly ,I don’t think gasket is only the problem , cause leak is really small and can’t lead to such sudden air lose . Need to look somewhere else but can’t see or hear air disappear.